Monday, May 10, 2010

Quick Facts About Soy and Health

Soy foods have been studied a great deal for their potential protection against a range of chronic conditions, including breast and prostate cancer, heart disease, osteoporosis, and menopausal symptoms.

It’s not known exactly how a diet rich in soy results in such benefits. It could be due to some of the individual components of soy, such as isoflavones which have some qualities similar to naturally occurring hormones. Or it could be the collective make up of soy, all its components together, which have the beneficial effects.

Because most pre-menopausal symptoms are linked to dropping levels of the hormone estrogen, it’s been considered that soy, and the insoflavones it contains, could help mimic natural estrogen and therefore quell things like hot flashes. Current evidence, though, suggests that soy intake actually has little impact on hormone levels. Still, research also suggest that a diet rich in soy may reduce menopausal symptoms (1).

The potential cardiovascular benefits from higher soy intake may be due to soy’s being a healthier replacement for some higher fat choices. Soy has only about 20 percent of its calories from fat, which is predominantly “healthy” polyunsaturated fat (2), whereas chicken can have nearly half of its calories from fat, and beef can have as much as 80 percent of its calories from fat, much of it in the form of “unhealthy” saturated fat. Studies looking at the relationship between soy intake and heart disease suggest that a diet rich in soy can improve blood pressure, blood cholesterol, and insulin levels (3).

Much of the interest in the links between soy intake and cancer risk is motivated by the historically low breast and prostate cancer risk among Asians, whose diets are traditionally high in soy. Detailed reviews by Wu and colleagues shows that at high intakes typical of Asian diets, soy is significantly related to a reduced risk for breast cancer, and the effect may be strongest for high intake in childhood and adolescence. A meta-analysis found that diets with high amounts of soy (20 mg per day of isoflavone) in Asian women was associated with a decreased risk for breast cancer, compared to Asian women consuming lower amounts (5 mg daily) (4). What this means for most American women is unclear. Studies in the US have typically not found a link between soy intake and breast cancer, but this could largely be due to widely different diets between the two regions. The highest intake in the US is typically 80 percent below the lowest intake in Asia (0.3 mg isoflavones/day in the US versus 25-50 mg isoflavones/day in Asia, which is equivalent to 1 g of soy food versus 80-160 g of soy food). Should diets shift to resemble those of Asia, it’s more than likely that studies would begin to reveal breast cancer benefits with soy as well.

Looking at the data on soy and prostate cancer, a number of studies show benefits from a higher intake of soy and soy-products. Soy milk intake was shown to be inversely related to risk of prostate cancer among US men in the Seventh Day Adventist Cohort study (5). As soy milk consumption went up, the risk of prostate cancer went down. A similar relationship has been seen between total soy intake and the risk of prostate cancer in other cohort and case-control studies (6), even though the amount of the effect on risk varied a bit from study to study. The only potential evidence that soy might increase the risk of prostate cancer comes from research of fermented soy (Miso), which has been positively related to risk in some studies.

Overall, there is currently little evidence that a diet high in soy is bad for health, and a growing body of evidence that it could have substantial health benefits. Realizing such potential benefits though, would likely take intake levels beyond what most people in the US typically eat.